I found an antecedent of the Government of Canada's advice in an Ontario high-school textbook from the 1920s. The book is called High-School English Composition. (H. W. Irwin and J. F. van Every [Toronto: The Copp Clark Company, 1921, rpt. 1929]) The authors say that the serial comma should be used only when necessary. To exemplify its capacity to alter one's meaning, they give this example:

During the Great War, when the British troops were engaged in a critical struggle with the Germans for the possession of Hill 70, General French sent the following message to England: 'We captured the western outskirts of Bulluch [sic., should be Hulluch], the village of Loos, and the mining works around it and Hill 70.' By an error, the message was made to read: 'We captured the western outskirts of [H]ulluch, the village of Loos, and the mining works around it, and Hill 70.' The insertion of the comma after 'it' conveyed the impression that Hill 70 had been captured. In consequence, public celebrations and rejoicings were held in all parts of the country. (pp. 199-200)

There has been a misunderstanding on this point. The message from Sir John French, which was published in the papers of Sept. 27, stated that we had captured the western outskirts of Hulluch village, Loos and the mining works around it and Hill 70. This was been [sic.] read to mean that Hill 70 had been taken. If the words were correctly read it would be seen that the capture only of the mining works around Hill 70 was claimed.